Solid-Phase Synthesis of Surfactin, a Powerful Biosurfactant Produced by Bacillus subtilis, and of Four Analogues

Interdisciplinary Overview of Lipopeptide and Protein-Containing Biosurfactants

Biosurfactants are amphipathic molecules capable of lowering interfacial and superficial tensions. Produced by living organisms, these compounds act the same as chemical surfactants but with a series of improvements, the most notable being biodegradability. Biosurfactants have a wide diversity of categories. Within these, lipopeptides are some of the more abundant and widely known. Protein-containing biosurfactants are much less studied and could be an interesting and valuable alternative. The harsh temperature, pH, and salinity conditions that target organisms can sustain need to be understood for better implementation. Here, we will explore biotechnological applications via lipopeptide and protein-containing biosurfactants. Also, we discuss their natural role and the organisms that produce them, taking a glimpse into the possibilities of research via meta-omics and machine learning.

Anti-proliferative activity of surfactins on human cancer cells and their potential use in therapeutics

Surfactins are cyclic lipopeptides that are isolated from various Bacillus strains. They are made up of heptapeptides and β-hydroxy fatty acids of variable chain lengths of carbon atoms. Therapeutically they are known to inhibit invasion, migration, and colony formation of human breast carcinoma cells. The role of surfactins is also known as anti-proliferative agents against human cancer cells through induction of apoptosis, arrest of the cell cycle, or suppression of survival signaling. The cytotoxic activity of surfactins is also perceived against human chronic myelogenous leukemia cells, human colon cancer cells, and hepatic carcinoma cells. Considering the wide spectrum of targets, the molecular effects of surfactins are diverse in different cancer cells and they can serve as promising chemotherapeutic agents for the treatment of cancer. Surfactins are being delivered to the targeted cancer cells through nano-carriers or nano-formulations. The present review article provides insight on different types and variations of surfactins, their molecular effect on different cancer cells, and their therapeutic use in the treatment of human cancer.

The Surfactin-Like Lipopeptides From Bacillus spp.: Natural Biodiversity and Synthetic Biology for a Broader Application Range

Surfactin is a lipoheptapeptide produced by several Bacillus species and identified for the first time in 1969. At first, the biosynthesis of this remarkable biosurfactant was described in this review. The peptide moiety of the surfactin is synthesized using huge multienzymatic proteins called NonRibosomal Peptide Synthetases. This mechanism is responsible for the peptide biodiversity of the members of the surfactin family. In addition, on the fatty acid side, fifteen different isoforms (from C12 to C17) can be incorporated so increasing the number of the surfactin-like biomolecules. The review also highlights the last development in metabolic modeling and engineering and in synthetic biology to direct surfactin biosynthesis but also to generate novel derivatives. This large set of different biomolecules leads to a broad spectrum of physico-chemical properties and biological activities. The last parts of the review summarized the numerous studies related to the production processes optimization as well as the approaches developed to increase the surfactin productivity of Bacillus cells taking into account the different steps of its biosynthesis from gene transcription to surfactin degradation in the culture medium.

Isolation and characterization of a new cyclic lipopeptide surfactin from a marine-derived Bacillus velezensis SH-B74

A marine-sediment-derived bacterium Bacillus velezensis SH-B74 can produce cyclic lipopeptides (CLPs). This study presented the isolation, characterization, and activity evaluation of a new CLP from the bacterial cultures of the strain SH-B74. Multiple chromatographic methods (solid-phase extraction and reversed-phase high-performance liquid chromatography) were applied to the purifying procedure of CLP, and the structural characterization of the new CLP was conducted by various spectroscopy (1D and 2D nuclear magnetic resonance together with Fourier transform infrared spectroscopy) and spectrometry (liquid chromatography-mass spectrometry, high-resolution mass spectrometry and tandem mass spectrometry) techniques as well as Marfey's method. The results displayed that the new CLP (anteiso-C₁₅ Ile_2,7 surfactin, 1) consists of a peptidic backbone of L-Glu₁, L-Ile₂, D-Leu₃, L-Val₄, L-Asp₅, D-Leu₆, L-Ile₇, and an anteiso-C₁₅ type saturated fatty acid chain. Further activity assay showed that the new CLP displays activity on the inhibition of the appressoria formation of rice blast causal pathogen Magnaporthe oryzae. To sum up, the results presented the perspective of potential application of the new CLP as a green agrichemical to control M. oryzae.

Structure elucidation of bacterial nonribosomal lipopeptides

We provide a summary of the tools, which allow elucidate the structures of nonribosomal lipopetides.

Bacilotetrins A and B, Anti-Staphylococcal Cyclic-Lipotetrapeptides from a Marine-Derived Bacillus subtilis

LC-MS and NMR spectroscopy guided metabolic profiling and dereplication of a crude extract obtained from the fermentation of a marine-derived bacterium, Bacillus subtilis, followed by chromatographic isolation yielded two new cyclic-lipotetrapeptides, bacilotetrins A (1) and B (2). Based on extensive 1D and 2D NMR and high-resolution ESIMS data analysis, the structures of 1 and 2 were elucidated, revealing the unique structures of these lipopeptides consisting of three leucines and a glutamic acid residue cyclized with a lipophilic 3-hydroxy fatty acid. The absolute stereochemistries at selected stereocenters in 1 and 2 were assigned by chemical derivatization and comparison to literature data. Compounds 1 and 2 exhibited anti-MRSA activity with MIC values of 8 to 32 μg/mL. However, these compounds showed no cytotoxicity when tested against prostate and liver cancer cell lines using the standard SRB assay.

A multicomponent macrocyclization strategy to natural product-like cyclic lipopeptides: synthesis and anticancer evaluation of surfactin and mycosubtilin analogues

Two birds in one shot: oligopeptides can be cyclized and lipidated in one step with multicomponent reactions.

Gageopeptins A and B, new inhibitors of zoospore motility of the phytopathogen Phytophthora capsici from a marine-derived bacterium Bacillus sp. 109GGC020

The motility of zoospores is critical in the disease cycles of the peronosporomycetes that cause devastating diseases in plants, fishes, vertebrates, and microbes. In the course of screening for secondary metabolites regulating the motility of zoospores of Phytophthora capsici, we discovered two new inhibitors from the ethyl acetate extract of the fermentation broth of a marine-derived strain Bacillus sp. 109GGC020. The structures of these novel metabolites were elucidated as new cyclic lipopeptides and named gageopeptins A (1) and B (2) by spectroscopic analyses including high resolution MS and extensive 1D and 2D NMR. The stereoconfigurations of 1 and 2 were assigned based on the chemical derivatization studies and reviews of the literature data. Although compounds 1 and 2 impaired the motility of zoospores of P. capsici in dose- and time-dependent manners, compound 1 (IC50 = 1 μg/ml) was an approximately 400-fold stronger motility inhibitor than 2 (IC50 = 400 μg/ml). Interestingly, the zoospores halted by compound 1 were subsequently lysed at higher concentrations (IC50 = 50 μg/ml). Compounds 1 and 2 were also tested against some bacteria and fungi by broth dilution assay, and exhibited moderate antibacterial and good antifungal activities.

Structural diversity of the microbial surfactin derivatives from selective esterification approach

Surfactin originated from genus Bacillus is composed of a heptapeptide moiety bonded to the carboxyl and hydroxyl groups of a β-hydroxy fatty acid and it can be chemically modified to prepare the derivatives with different structures, owing to the existence of two free carboxyl groups in its peptide loop. This article presents the chemical modification of surfactin esterified with three different alcohols, and nine novel surfactin derivatives have been separated from products by the high performance liquid chromatography (HPLC). The novel derivatives, identified with Fourier transform infrared spectroscopy (FT-IR) and electrospray ionization mass spectrometry (ESI-MS), are the mono-hexyl-surfactin C14 ester, mono-hexyl-surfactin C15 ester, mono-2-methoxy-ethyl-surfactin C14 ester, di-hexyl-surfactin C14 ester, di-hexyl-surfactin ester C15, di-2-methoxy-ethyl-surfactin ester C14, di-2-methoxy-ethyl-surfactin ester C15, di-6-hydoxyl-hexyl-surfactin C14 ester and, di-6-hydoxyl-hexyl-surfactin C15 ester. The reaction conditions for esterification were optimized and the dependence of yields on different alcohols and catalysts were discussed. This study shows that esterification is one of the most efficient ways of chemical modification for surfactin and it can be used to prepare more derivatives to meet the needs of study in biological and interfacial activities.

Non-cytotoxic antifungal agents: isolation and structures of gageopeptides A-D from a Bacillus strain 109GGC020

Antifungal resistance and toxicity problems of conventional fungicides highlighted the requirement of search for new safe antifungal agents. To comply with the requirement, we discovered four new non-cytotoxic lipopeptides, gageopeptides A-D, 1-4, from a marine-derived bacterium Bacillus subtilis. The structures and stereochemistry of gageopeptides were determined by NMR data analysis and chemical means. Gageopeptides exhibited significant antifungal activities against pathogenic fungi Rhizoctonia solani, Botrytis cinerea, and Colletotrichum acutatum with minimum inhibitory concentration (MIC) values of 0.02-0.06 μM. In addition, these lipopeptides showed significant motility inhibition and lytic activities against zoospores of the late blight pathogen Phytophthora capsici. These compounds also showed potent antimicrobial activity against Gram positive and Gram negative bacteria with MIC values of 0.04-0.08 μM. However, gageopeptides A-D did not exhibit any cytotoxicity (GI50 > 25 μM) against cancer cell lines in sulforhodamine B (SRB), 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT), and WST-1 ((4-[3-4-iodophenyl]-2-(4-nitrophenyl)-2H-5-tetrazolio)-1,3-benzene disulfonate)) assays, demonstrating that these compounds could be promising candidates for the development of non-cytotoxic antifungal agents.

Gageostatins A-C, antimicrobial linear lipopeptides from a marine Bacillus subtilis

Concerning the requirements of effective drug candidates to combat against high rising multidrug resistant pathogens, we isolated three new linear lipopeptides, gageostatins A–C (1–3), consisting of hepta-peptides and new 3-β-hydroxy fatty acids from the fermentation broth of a marine-derived bacterium Bacillus subtilis. Their structures were elucidated by analyzing a combination of extensive 1D, 2D NMR spectroscopic data and high resolution ESIMS data. Fatty acids, namely 3-β-hydroxy-11-methyltridecanoic and 3-β-hydroxy-9,11-dimethyltridecanoic acids were characterized in lipopeptides 1 and 2, respectively, whereas an unsaturated fatty acid (E)-7,9-dimethylundec-2-enoic acid was assigned in 3. The 3R configuration of the stereocenter of 3-β-hydroxy fatty acids in 1 and 2 was established by Mosher’s MTPA method. The absolute stereochemistry of amino acid residues in 1–3 was ascertained by acid hydrolysis followed by Marfey’s derivatization studies. Gageostatins 1–3 exhibited good antifungal activities with MICs values of 4–32 µg/mL when tested against pathogenic fungi (R. solani, B. cinerea and C. acutatum) and moderate antibacterial activity against bacteria (B. subtilis, S. aeureus, S. typhi and P. aeruginosa) with MICs values of 8–64 µg/mL. Futhermore, gageostatins 1–3 displayed cytotoxicity against six human cancer cell lines with GI₅₀ values of 4.6–19.6 µg/mL. It is also noteworthy that mixed compounds 1+2 displayed better antifungal and cytotoxic activities than individuals.

Characterization of an antifungal compound produced by Bacillus sp. strain A(5) F that inhibits Sclerotinia sclerotiorum

A potential antagonist, Bacillus sp. strain A(5) F was isolated from soybean rhizosphere following in vitro dual plate screening. The bacterium displayed strong inhibitory activity in vitro against soybean stem rot pathogen, Sclerotinia sclerotiorum. The culture supernatant of strain A(5) F completely suppressed the mycelial growth of the pathogen, indicating that suppression was due to the presence of antifungal compounds in the culture filtrate. The culture filtrate also suppressed other phytopathogenic fungi including Fusarium oxysporum and Macrophomina phaseolina, in vitro suggesting a broad spectrum antagonistic activity against fungal pathogens. Chemical extraction followed by chromatographic analysis resulted in two antifungal fractions. The high resolution-electron spin ionization-mass spectrometry (HR-ESI-MS) and Nuclear Magnetic Resonance (1D and 2D(1) H) spectra of these antifungal fractions revealed the presence of antifungal compounds, one of which showed similarity to bacillomycin D.

Lipopeptides in cosmetics

Lipopeptides are biosurfactants extensively used in cosmetics. The consumption of cosmetics containing lipopeptides is increasing as a result of the exceptional surface properties and diverse biological activities of lipopeptides which facilitate a vast number of applications not only in the pharmaceutics industry which includes cosmetics but also in the food industry. Cosmetics containing lipopeptides are available in various dosage forms according to their beneficial surface properties, which include anti-wrinkle and moisturizing activities and cleansing cosmetics. The microbial production of lipopeptides particularly those with biological and surface activities applicable to cosmetics are summarized based on appropriate studies and patents up to the year 2008 to manage the information and sufficiently review the data.

Molecular and biochemical characterization of Iranian surfactin-producing Bacillus subtilis isolates and evaluation of their biocontrol potential against Aspergillus flavus and Colletotrichum gloeosporioides

The characterization of surfactin-producing Bacillus subtilis isolates collected from different ecological zones of Iran is presented. Characterization was performed using blood agar, PCR, drop-collapse, and reverse-phase high-performance liquid chromatography (HPLC) analyses, and the isolates' biocontrol effects against the aflatoxin-producing agent Aspergillus flavus and the citrus antracnosis agent Colletotrichum gloeosporioides were studied. In total, 290 B. subtilis isolates were isolated from phylosphere and rhizosphere samples collected from fields and gardens of 5 provinces of Iran. Blood agar assays showed that 185 isolates produced different biosurfactants. Isolates containing the sfp gene, coding for surfactin, were detected using the PCR method. It was found that 14 different isolates contained the sfp gene. Drop-collapse assays, which detect isolates with high production of surfactin, showed that 7 isolates produced high levels of surfactin. It was found from HPLC analysis that the isolates containin the sfp gene produced between 55 and 1610 mg of surfactin per litre of broth medium. Four isolates, named BS119m, BS116l, N3dn, and BS113c, produced more than 1000 mg of surfactin per litre of broth. The highest surfactin production level was observed for isolate BS119m (1610 mg/L). The antagonistic potential of the sfp gene-containing isolates was determined using dual culture and chloroform vapour methods. Our bioassay results indicated that isolate BS119m showed high inhibitory effects against A. flavus (100%) and C. gloeosporioides (88%). Furthermore, the effect of purified surfactin on the growth of A. flavus was evaluated. Mycelia growth was considerably reduced with increasing concentration of surfactin, and 36%, 54%, 84%, and 100% inhibitions of mycelia growth were, respectively, observed at 20, 40, 80, and 160 mg/L after 7 days of incubation.

Aggregation of the naturally occurring lipopeptide, surfactin, at interfaces and in solution: an unusual type of surfactant?

Neutron reflectometry has been used to study the structure of the biosurfactant, surfactin, at the air/water and at the hydrophobic solid/water interfaces. Three different deuterated surfactins were produced from the Bacillus subtilis strain: one perdeuterated, one with the four leucines perdeuterated, and one with everything except the four leucines perdeuterated. The neutron reflectivity profiles of these three samples in null reflecting water and in D20 with a seventh profile of the protonated surfactin in D2O were measured at pH 7.5. This combination of different isotopic compositions made it possible to deduce the distribution of each type of labeled fragment in the surfactin. Surfactin is found to adopt a ball-like structure with a thickness of 14 +/- A and an area per molecule of 147 +/- 5 A2. This makes it more like a hydrophobic nanoparticle, whose solubility in water is maintained only by its charge, than a conventional surfactant. This is probably what makes it surface-active at such low concentrations and what contributes to its forming very compact surface layers that are more dense than observed for most conventional amphiphiles. The reflectivity data were fitted by a model in which the structure of surfactin was divided into three fragments: the four leucines taken as a group, the hydrocarbon chain, and a hydrophilic group containing the two negative charges. An analysis of the reflectivity gave the following separations between fragments, where zero corresponds to the Gibbs plane for zero water adsorption: chain-water 7 A, hydrophile-water 1 A, and leucines-water 6.5 A, all +/- 1 A. The overall structure of the layer appears to be identical at a hydrophobic octadecyltrichlorosilane-coated silicon surface where the thickness of the surfactin layer is 15 +/- 1 A and the area per molecule is 145 +/- 5 A2. Finally, the structure of surfactin micelles has been examined by means of small-angle neutron scattering. The aggregation number was found to be unusually small at 20 +/- 5. The structure of the micelle is of the core-shell type with the hydrocarbon chain and the four hydrophobic leucines forming the core of the micelle.